The present invention relates to a laser modulating device employed in, for example, a laser scanning device, and particularly to a laser modulating device equipped with for modulating the output power of the laser the laser diode.
In a laser scanning device having a laser diode as a light source, a laser modulating device is generally provided. The laser modulating device modulates the output power of the laser diode while the emitted beam is deflected to scan on a photoconductive surface. An image having a black density distribution corresponding to modulated level of the intensity of the laser beam is formed on the photoconductive surface.
In such a device, electrical currents corresponding to a minimum black density level, i.e., the white level and a maximum black density level are firstly determined. Then, the laser diode is driven such that an electrical current whose level is between the above-described white level and black level currents is supplied. In order to compensate for the intensity of the emitted beam due to individual characteristic errors of laser diodes and/or change of ambient temperature, part of light emitted by the laser diode is monitored, and a feed-back control which is known as an APC (automatic power control) is executed.
FIG. 6 shows a block diagram of an example of a conventional modulating circuit employing the APC system. A D/A (digital to analog) converter 21 outputs a data voltage Vdata corresponding to an input data signal Sdata representative of black density level of an image. The Sdata is generated based on the maximum level voltage Vmax corresponding to maximum output power Pmax of a laser diode LD, which is transmitted from a maximum level setting circuit 22.
The data voltage Vdata is added to a minimum level voltage Vmin by an adder 24. The minimum level voltage Vmin is a voltage for setting the minimum output power Pmin of the laser diode LD, which is transmitted from a minimum level setting circuit 23. With this structure, the adder 24 outputs a reference voltage Vref which is a sum of the minimum level voltage Vmin and the data voltage Vdata. The reference voltage Vref is input to an input terminal of a differential amplifier 25, and is compared with the monitor voltage Vmon, which is input to the other input terminal of the differential amplifier 25. The differential amplifier outputs a voltage Vdef, which is an amplified voltage of a difference between the monitor voltage Vmon and the reference voltage Vref. The voltage Vdef is converted into an electrical current corresponding thereto by a V/I (voltage to current) conversion circuit 26. The electrical current is supplied to the laser diode LD as a driving current Id. The laser beam emitted by the laser diode LD is received by a photo diode PD for monitoring, which receives part of the laser beam (generally a beam backwardly emitted by the laser diode LD) and outputs a monitoring current Imon. The monitoring current Imon is converted into the monitor voltage Vmon by an I/V (current to voltage) conversion circuit 27.
In this modulating device, depending on a data value of the data signal Sdata, the data voltage Vdata output by the D/A conversion circuit 21 varies. Therefore, the reference voltage Vref varies in accordance with the data signal Sdata since the reference voltage Vref is generated based on the data voltage Vdata. If the intensity of the light beam emitted by the laser diode LD is smaller than the intensity corresponding to the value of the data signal Sdata, the monitoring current Imon is relatively small, and therefore, the voltage Vmon output by the I/V conversion circuit 27 is small. In this case, the differential amplifier 25 outputs a relatively large voltage Vdef, and therefore, the driving current Id output by the V/I conversion circuit 26 is increased, and the intensity of the light beam emitted by the laser diode LD is increased. If the intensity of the laser beam is greater than an intensity corresponding to the reference voltage Vref, the monitor voltage Vmon is greater than the reference voltage Vref. Then, the output voltage Vdef of the differential amplifier 25 is lowered. Thus, the driving current Id output by the V/I conversion circuit 26 is lowered, and the intensity of the light beam emitted by the laser diode LD is lowered.
With the above-described APC system, the intensity of the laser beam emitted by the laser diode LD is controlled to be an intensity corresponding to the reference voltage Vref. If the data voltage Vdata is changed as the data signal Sdata is changed, the reference voltage Vref is changed, and then the intensity of the laser beam output by the laser diode LD is changed accordingly. That is, the intensity of the laser beam emitted by the laser diode LD is modulated in accordance with the data signal Sdata. Thus, black density of an image formed on a photoconductive surface (not shown) is adjusted in accordance with the data signal Sdata.
In the above-described modulating device employing the APC system, when the data signal Sdata is changed and the reference voltage Vref is changed, the intensity of the laser beam emitted by the laser diode LD should be quickly changed responsive to the change of the reference voltage Vref. Therefore, the D/A conversion circuit 21, the I/V conversion circuit 27, the differential amplifier 25 and the V/I conversion circuit 26 should be constituted as quick-response (high-speed) circuits. However, in order to constitute a quick-response circuit, electronic elements and semi-conductor elements should be quick-responsive ones, which increases a manufacturing cost of each circuit. Further, even if the each circuit is constituted as a quick-responsive circuit, due to accumulated affects of potential delays of the I/V conversion circuit 27, differential amplifier 25 and V/I conversion circuit 26, it is still difficult to lower the delays sufficiently. Thus, it has been difficult to constitute a quick-response APC type modulating device.
It is therefore an object of the invention to provide an improved laser modulating device having a quick-response so that the delay within an APC loop is relatively small.
For the above object, according to the present invention, there is provided a laser modulating device for driving a laser diode to emit a modulated laser beam in accordance with input data. The laser modulating device is provided with a monitoring system that detects output power of the laser diode and outputs a detection signal corresponding to the output power, a minimum output level setting system that outputs a minimum level signal defining a minimum output power of the laser diode, a modulating signal generating system that generates a modulating signal representing modulation information of the output power of the laser diode corresponding to input data, a first adder that outputs a sum of the minimum level signal and the modulating signal as a reference signal, a comparing system that compares the reference signal and the detection signal and outputs the driving signal so as to make the detection signal coincide with the reference signal, and a circuit that supplies the modulating signal to the laser diode, the modulating signal being added to the driving signal.
With this configuration, the modulating signal is directly supplied to the laser diode at a first stage, the output power is adjust quickly. Further, the APC functions after the output power was adjusted, and therefore, the output power is stabilized. Furthermore, regardless of the amplitude of the modulating signal, the driving signal has substantially the same amplitude, which also stabilizes the output power of the laser diode.
Optionally, the modulating signal generating system may include a maximum output level setting system that outputs a maximum level voltage required for setting a maximum output power of the laser diode, and a modulating voltage generating circuit that generates, in accordance with the maximum level voltage, a data voltage whose level corresponds to a data value of the data signal.
Further, the modulating voltage generating circuit includes a D/A conversion circuit that outputs one of a plurality of levels of voltage values in accordance with a data value of the data signal.
Still optionally, the monitor system may include a photo diode that outputs an electrical current corresponding to a received light beam, and an I/V conversion circuit that converts the electrical current generated by the photo diode into a monitor voltage, Further, the reference signal is a reference voltage, and the comparing system includes a differential amplifier that compares the reference voltage with the monitor voltage, and outputs a voltage corresponding to the difference between the reference voltage and the monitor voltage. Further, and a first V/I conversion circuit that generates a first driving current based on the voltage output by the differential amplifier, the first driving current being supplied to the laser diode.
In this case, the circuit may include a V/I conversion circuit that generate a second driving current based on the data voltage, the second driving current being added to the first driving current.
Preferably, the D/A conversion circuit and the second V/I conversion circuit are high-speed circuits, respectively.
Still optionally, the circuit may include a gain adjustment circuit that adjusts the gain of the data voltage and outputs the modulating voltage, and a second adder that adds the modulating voltage output by the gain adjustment circuit to the voltage output by the differential amplifier, the added voltage being input to the first V/I conversion circuit.
In this case, it is preferable that the D/A conversion circuit, the gain adjustment circuit and the first V/I conversion circuit are high-speed circuits, respectively.
In a particular case, there is provided a laser modulating device for driving a laser diode to emit a modulated laser beam in accordance with input data, which is provided with a monitoring system that detects output power of the laser diode and outputs a detection voltage signal corresponding to the output power, a minimum output level setting system that outputs a minimum level voltage signal defining a minimum output power of the laser diode, a modulating voltage signal generating system that generates a modulating voltage signal representing modulation information of the output power of the laser diode corresponding to the input data, a first adder that outputs a sum of the minimum level voltage signal and the modulating voltage signal as a reference voltage signal, a comparing system that compares the reference voltage signal and the detection voltage signal, and outputs a driving voltage signal for the laser diode so as to make the detection voltage signal coincide with the reference voltage signal, a first voltage to current converting system that converts the driving voltage signal to a first driving current, the first driving current being supplied to the laser diode, a second voltage to current converting system, and a circuit that directly supplies the modulating voltage signal from the modulating voltage signal generating system to the second voltage to current converting system, the second voltage to current converting system converting the modulating voltage signal into a second driving signal, the second driving signal being supplied to the laser diode.
In another particular case, there is provided a laser modulating device for driving a laser diode to emit a modulated laser beam in accordance with input data, which is provided with a monitoring system that detects output power of the laser diode and outputs a detection voltage signal corresponding to the output power, a minimum output level setting system that outputs a minimum level voltage signal defining a minimum output power of the laser diode, a modulating voltage signal generating system that generates a modulating voltage signal representing modulation information of the output power of the laser diode corresponding to the input data, a first adder that outputs a sum of the minimum level voltage signal and the modulating voltage signal as a reference voltage signal, a comparing system that compares the reference voltage signal and the detection voltage signal, and outputs a differential voltage signal representing a difference between the reference voltage signal and the detection voltage signal, a gain adjustment system that converts the modulating voltage signal into a modified modulating voltage signal in accordance with a differential efficiency of the laser diode, a second adder that outputs a sum of the differential voltage signal and the modified modulating voltage signal and outputs a driving voltage signal, and a voltage-to-current converting system that converts the driving voltage signal to a driving current, the first driving current being supplied to said laser diode.